Nerve growth factor (NGF) is required for the survival and development of sympathetic, sensory, and certain central nervous system neurons. It binds to specific cell surface receptors on these neurons, initiates a chain of intracellular events and controls the expression of specific genes. The molecular mechanism(s) by which NGF controls gene expression are not known, but the intracellular events mediating the actions of the factor are becoming clear. We have found that the binding of NGF to its receptor is followed rapidly by a calcium-dependent activation of phosphoinositide metabolism and a calcium-independent increase in the release of arachidonic acid. These and other changes in second messenger levels lead to alterations in a number of protein kinases and, in turn, to changes in the phosphorylation of key cellular proteins. Among these are EF-2 and the S6 protein of the ribosomes. In this latter case, it has been found that NGF activates a different S6 kinase than does epidermal growth factor (EGF), a mitogen. This suggests that S6 plays a role in whether the cell divides or differentiates. Also altered is the phosphorylation of a nuclear protein (SMP) perhaps involved in regulating the transcription of specific genes. Work with cell- free systems suggested that there are a number of parallel, but largely independent phosphorylation cascades, all emanating from the receptor. A major effort now is to find that single biochemical reaction coupling the receptor to these several cascades. The search for this reaction is being conducted using a new tool, K-252a, developed in this laboratory. K-252a is a kinase inhibitor specific for the effects of NGF on PC12 cells. Other studies have focused on the changes in the expression of specific gene products that underlie NGF-induced differentiation. We have found that the expression of the cell recognition molecule Ng-CAM, known to be increased by NGF treatment, is prevented by the presence of glucocorticoids. The functional consequences of the absence of this cell membrane constituent on what are otherwise fully differentiated neurons are currently being explored. Finally, we are exploring the molecular basis of the NGF induced down-regulation of receptors for EGF, a mitogen for these cells. Our present data suggest that the decrease in mitogen receptors could be due to an NGF-induced alteration in the phosphorylation state of these receptors. This study may yield an important insight into the overall control of differentiation and cell division.